[electrum] add more docstrings and typehint to network and interface code

Summary:
This is an attempt at documenting the current behavior the network's state machine.

The fixme comment points to the codepath that causes the reorg issue in the tests in D14944.

Test Plan:
Mostly proof-reading.

Check also that we didn't accidentaly break anything:

python test_runner.py
pytest electrumabc/tests/regtest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14964